ANODIC FORMATION AND SEM CHARACTERIZATION OF ZIRCONIUM OXIDE NANOSTRUCTURED FILMS

Zirconium oxide (ZrO 2 ) is a widely utilized inorganic material with excellent dielectric, optical, and biocompatible characteristics. The properties and areas of the ZrO 2 applications can be further broadened by making the material nanostructured. In this work, anodic ZrO 2 nanostructured films were developed and characterized by scanning electron microscopy. The films were prepared by anodizing magnetron-sputtered Al/Zr bilayers in two regimes: galvanostatic/potentiostatic anodizing in (COOH) 2 and H 2 SO 4 solutions (anodized films) and that followed by the high-speed reanodizing to a significantly higher anodic voltage (reanodized films). The growth of a porous anodic alumina (PAA) layer followed by PAA-assisted oxidation of the Zr underlayer was achieved. The anodized films consist of arrays of self-organized spatially ordered ZrO 2 nanohillocks while the reanodized films comprise arrays of vertically aligned ZrO 2 nanorods. The growth of amorphous ZrO 2 nanohillocks and nanorods within the alumina pores via migration of Zr 4+ cations is a unique situation for anodic films on zirconium, which normally grow crystalline and by O 2- anion transport at the oxide/metal interface only. The achievement is a milestone towards understanding the ion transport during the PAA-assisted anodization of valve metals with low cation transport numbers


INTRODUCTION
Zirconium oxide (ZrO2) is a multifunctional inorganic compound highly utilized in many technological areas due to its remarkable properties such as high melting point, good mechanical and thermal resistance, chemical stability, low electrical conductivity, and biocompatibility. It has been employed in protective coatings, optoelectronic devices, sensors, fuel cells, catalysts, and medical implants [1][2][3][4][5].
Active steps have been made to further enhance the properties of the oxide through making it nanostructured. Commonly used methods include sol-gel and variety of gas-phase processes [6,7]. Relatively recently, an electrochemical approach (anodizing) has been developed as a simple and versatile technique for forming ZrO2 nanoporous films on top of the metal [8]. However, preparation of substrate-supported 1-D ZrO2 structures like nanoneedles, nanorods, and nanowires remains a challenge. Most recently, an alternative anodization technique has emerged for processing superimposed valve-metal films consisting of a layer of aluminum over a layer of different metal. The aluminum layer is first converted to porous anodic alumina (PAA), then the underlying metal is anodized through the alumina nanopores. Although the approach was a success for forming Ta, Nb, W, and Hf oxide nanostructures [9][10][11], the obstacles for growing zirconium-oxide nanostructures are the noticeably low Zr 4+ cation transport number (0.05) and the crystallization of the oxide under the high electric field [12,13].
Here we report the formation of self-ordered and uniformly aligned ZrO2 nanostructures via the PAA-assisted anodizing of Al/Zr bilayers on substrates. Due to the appropriate electrolytic and electrical conditions, a sustainable growth of nanostructured zirconium anodic oxide was achieved not only at the oxide/metal interface but also at the electrolyte/oxide interface, i.e. within the alumina nanopores. The films were examined by high-resolution scanning electron microscopy after selective dissolution of the PAA overlayer. For the first series of samples (anodized films) anodic process was performed in a constant current mode followed by a potentiostatic mode after the complete consumption of the aluminum layer. The second series of samples (reanodized films) were prepared by anodizing at the same conditions followed by a rapid rise in the applied voltage to a significantly higher value (reanodizing), in order to promote the growth of zirconium oxide within the PAA pores. After the anodizing and reanodizing, the samples were thoroughly rinsed with deionized water and dried in an oven at 120 °C for 10 minutes. In order to selectively dissolve the PAA overlayer (PAA-free films), the samples were dipped in a stirred solution of 0.45М H3PO4 and 0.2М Cr2O3 heated to 65 °C, thoroughly washed in deionized water, and dried in air at 120 °C for 10 minutes.

Sample characterization
The surface morphology and cross fractures of the PAA-free samples were examined in a TESCAN MIRA II field-emission scanning electron microscope, equipped with an InBeam detector for secondary electrons, without depositing any conducting layer.

Anodizing behavior
A typical electrochemical response during anodizing, followed by reanodizing in 0.2M (COOH)2 is shown in Figure 1. The first processing stage is the same for both the anodized and reanodized samples. When a constant current density of 10 mA‧cm -2 is applied, the voltage begins to gradually increase at a rate of about 1.7 V⋅s -1 due to the growth of a compact anodic film on the Al layer. After reaching a maximum of about 61 V, the voltage begins to gradually decrease until it attains a steady-state value of approximately 50 V at the end of stage I. Such voltage behavior reflects the nucleation and growth of a porous anodic film. At the commencement of stage II, a steady-state PAA growth begins, then the voltage remains stable. After about 90 seconds of the stable PAA growth, the aluminum layer is almost fully consumed and the bottoms of growing PAA cells reach the Zr metal (the end of stage II). Then the voltage is allowed to slightly grow up to a set point of 55 V, after that the setup is automatically switches to a potentiostatic mode, preventing the oxide from further growth. In about 60 s of voltage stabilization, the current density decreases to an almost steady low value. The voltage-time behavior during the anodic process is typical for PAA growth, so one could expect a good degree of pore self-ordering in the upper PAA layer [14].
During stage III, a different approach is implemented for forming the reanodized samples (Figure 1). After a 60-second current decay after beginning of stage III, the voltage is rapidly raised at a rate of 10 V⋅s -1 (sweep rate) to 250 V (reanodizing voltage) and is stabilized for 120 s. During the voltage sweep, the current density increases rapidly to a value of about 9 mA‧cm -2 and remain relatively stable until the voltage reaches the set point.
During the voltage stabilization period at stage III, the current drops to an almost steady and substantially low value, then the polarization is cut off. The electrochemical responses for anodizing and reanodizing other samples in all the solutions correlated with the behavior shown in Figure 1, the exact process variables and morphological features being collected in Table 1.   From the tilted images (Figures 3 e, f, g, h), it is seen that the ZrO2 nanostructures resemble rods, which are substantially longer than the hillocks grown in the anodized samples and rise well above the general film level. An important feature of the reanodized samples is that some tips of the nanorods tend to aggregate with their closest neighbors. These phenomena might be caused by a combination of two reasons. First, the nanostructures of such aspect ratio are the subject of strong van der Waals forces. Second, the bases of the nanorods might contain an amount of residual alumina mixed with zirconium oxide and partly dissolved during the PAA removal process, which results in mechanically weakening the bases. The latter might be resolved through further optimization of reanodizing conditions and will be studied in a future work.
Cross-section SEM images of the reanodized samples (Figure 4) allow to estimate the form and size of the ZrO2 nanostructures. The length and diameter of the rods vary depending on anodizing electrolyte. Further, the shape of the rods is not uniform along their length, and the diameters widen towards the substrate level, resembling the form of a tree trunk. It may be assumed that the hillocks are of the same sizes as the bottom parts of the rods and, therefore, might have similar chemical composition. This implies that the narrower, tall part of the rods is ZrO2 that forms by filing the pores in the PAA upper layer during the reanodizing process. From Figure 4, the nanorods seem to be well separated from the remaining Zr metal by a thin oxide layer, which is believed to be crystalline ZrO2. The SEM results confirm that ZrO2 can grow through the alumina barrier layer and further within the pores of the upper PAA film, despite the commonly known fact that the oxide is only expected to grow in crystalline form underneath the alumina barrier layer [15]. This may be explained by suggesting that during the anodizing process amorphous ZrO2 grows towards the pore space by migration of Zr 4+ through the alumina barrier layer and simultaneously mixing with alumina within the barrier layer. The ZrO2 hillocks thus formed inside the barrier layer can provide the pathway for further oxidation of Zr during the reanodizing. When reanodizing proceeds, the pores of the upper PAA layer are filled with growing ZrO2 material and form rod-like nanostructures. From such behavior, it is obvious that the transport number of Zr 4+ cation through the oxide becomes significantly higher than 0.05, the phenomenon to be investigated in more detail in future works.

CONCLUSION
Upright standing self-ordered arrays of ZrO2 nanostructures may be fabricated by anodization of Al/Zr bilayered films in (COOH)2 and H2SO4 electrolytes at various formation voltages. Generally, the process involves the conversion of Al metal into a PAA layer followed by oxidation of nether Zr layer. The growth of PAA-assisted hillock-like nanostructures of various sizes and geometries is achieved for the first time. The lengthening of ZrO2 nanostructures within the PAA pores appeared to be also possible due to the high-speed reanodizing technique. Substrate-supported self-ordered arrays of ZrO2 nanohillocks or nanorods are obtained after the dissolution of the PAA layer. The ZrO2 nanostructures thus grown are supposedly amorphous in their nature. Their formation is believed to be driven by an extraordinary increased outward migration of Zr 4+ cations through the alumina barrier layer during the constant-current anodizing and high-speed reanodizing processes. This study contributes to understanding and utilizing the unusual ion migration and solid-state reactions during the PAA-assisted anodization of valve metals having basically low cation transport numbers.